Heretofore, there have been developed vibration sensors using an electret film. As shown in FIG. 1, a vibration sensor includes a closed case 30 containing a back plate 31 with an electret film formed thereon. A diaphragm 32 opposed to the electret film of the back plate 31 is disposed with a very small clearance between it and the back plate. The back plate 31 is fixed within the case 30. Electric charges are accumulated between the electret film of the back plate 31 and the diaphragm 32. In this case, the diaphragm 32 is formed of a thin metal plate and is supported so as to be easily vibrated. In principle, the vibration sensor shown in FIG. 1 senses how much electric field changes when the diaphragm 32 is vibrated. The result is a voltage signal that indicates an amount of vibrations applied to the sensor.
Specifically, if the diaphragm 32 is fixed, then the vibration sensor is vibrated together with a monitored object to which the vibration sensor is attached. When the vibration sensor is vibrated together with the monitored object, the back plate 31 is vibrated relative to the diaphragm 32. As a result, the electrostatic capacity between the back plate 31 and the diaphragm 32 is changed based on vibrations of the back plate 31. The detected change of the electrostatic capacity is output by the vibration sensor. Although it is considered that the diaphragm 32 is fixed, if the vibration sensor is fixed together with the detected object, then it is natural that the diaphragm 32 should be vibrated based on vibrations applied to the vibration sensor. In any event, the vibration sensor detects vibrations applied to the monitored object from the outside or vibrations generated in the monitored object as the change of electrostatic capacity accumulated between the diaphragm 32 and the back plate.
The back plate 31 and the diaphragm 32 are accommodated within the closed case 30 in order to prevent the diaphragm 32 from being vibrated by a sound pressure based on sounds applied to the vibration sensor from the outside so that the vibration sensor is able to detect only vibrations accurately. Specifically, if the diaphragm 32 is vibrated in response to sounds applied to the sensor, then the vibration sensor registers vibrations together with sound and the output from the vibration sensor contains an error component. In order to prevent this defect, the back plate and the diaphragm should be almost completely enclosed by the case 30. FIG. 2 is a graph showing a sound pressure frequency characteristic of the closed type vibration sensor. Since the case 30 is of the closed type, the vibration sensor responds to only a sound or sound pressure having a relatively low frequency and is unable to respond to a sound or sound pressure having a frequency higher than a certain frequency. When the vibration sensor is applied to a navigation apparatus in a vehicle, the vibration sensor tends to detect sounds generated in the vehicle, e.g., an automobile. There are then the problems that a position on a correct map cannot be displayed and that a correct position cannot be displayed on a map due to the error component from the vibration sensor.
Study of characteristics shown in FIG. 2 reveals that the sensor inevitably responds to the sound pressure of low frequency sounds applied to the vibration sensor and generates an erroneous output. For this reason, it is proposed to form a through-hole on the diaphragm. Specifically, when a response of the diaphragm relative to a sound pressure is in the state shown by the solid line in FIG. 3, if a through-hole is defined at the central portion of the diaphragm, then a minimum frequency response to sound pressure can be shifted to a higher frequency as shown by the broken line in FIG. 3. The minimum frequency response to sound pressure can be further shifted to a higher frequency as an area of the through-hole increases.
If the minimum frequency response to the sound pressure generated based on sounds applied to the diaphragm from the outside is raised by using a through-hole as described above, as shown in FIG. 2, then the diaphragm responds to only a sound pressure having a frequency higher than a band responsive to the sound pressure of the low frequency shown by the sound pressure frequency characteristic of the closed-type vibration sensor. As a result, the diaphragm responds only to vibrations applied to the sensor and the sensor can indicate those vibrations by the output signal.
If however the through-hole is defined at the central portion of the diaphragm to lower sensitivity to sound pressure with a low band frequency, there is then the disadvantage that the detection sensitivity of the vibration sensor is lowered. Specifically, the level at which the diaphragm without a through-hole detects vibrations shown by the solid line in FIG. 4. Then when the through-hole is provided in order to lower sensitivity to sound pressure of the low frequency, the output level of the detection signal is lowered as shown by the broken line in FIG. 4. The output level is lowered much more as the area of the through-hole is increased. That is, the through-hole decreases an area in which the diaphragm and the back plate oppose each other, lessening a usability of electric charges of the electret film. As a result, the output level of the detection signal is lowered.
Therefore, this kind of conventional vibration sensor cannot raise a vibration detection sensitivity to a desirable extent.